The present invention relates to a roller bearing, a vehicle-use transmission and a differential gear.
A tapered roller bearing is provided with an outer ring, an inner ring and a plurality of tapered rollers interposed between the outer ring and the inner ring. Collar portions having guide surfaces guiding both end faces in a large diameter side and a small diameter side of the tapered roller in accordance with a slide contact are provided in both end portions of the inner ring in an axial direction. These elements are lubricated by a lubricant such as an oil, a grease or the like. Surface roughness of a contact surface between the end face of the tapered roller and the guide surface of the collar portion of the inner ring exerts an influence on a lubricating property.
A description will be given of the lubricating property with reference to FIG. 15. Reference symbol 4A denotes a large diameter side end face of a tapered roller, and reference numeral 8 denotes a guide surface in a collar portion of an inner ring. In conventional tapered roller bearings, surface roughness of the large diameter side end face 4A of the tapered roller is generally set in which surface roughness in a diametrical direction is higher than surface roughness in a peripheral direction due to the direction of a grinding work.
On the other hand, in a tapered roller bearing used in a vehicle-use transmission, a vehicle-use differential gear or the like, a preload applied at a time of assembling is controlled by a torque under a low speed rotation. When a dispersion of the low speed rotation torque (an assembling torque) is wide, there is developed problems such that an early seizure is caused by an excessive preload, a rigidity reduction is caused by an insufficient preload, and the like. Accordingly, in order to apply a suitable preload to the tapered roller bearing, it is required that the dispersion of the assembling torque is small and a fluctuation thereof is small.
Most of the assembling torque of the tapered roller bearing is caused by friction between the guide surface 8 and the large diameter side end face 4A of the tapered roller in the collar portion of the inner ring 4. Accordingly, a lubricating property such as surface roughness of the guide surface 8 and the large diameter side end face 4A, an oil film thickness formed between them or the like exerts a great influence on a friction coefficient, that is, a torque. The surface roughness of the large diameter side end face 4A of the tapered roller is set so as to inhibit the dispersion of the rotation torque and apply a proper preload, however, a treatment method of the surface exerts a great influence on the surface roughness.
If the surface roughness of the large diameter side end face 4A of the tapered roller is set to this aspect, an oil film formation becomes unstable in the slip contact portion between the large diameter side end face 4A of the tapered roller and the guide surface 8 in the collar portion of the inner ring under the low speed rotation, the dispersion of the rotation torque (the assembling torque) becomes great, and an amount of lubricating oil becomes insufficient in a contact portion A between the large diameter side end face 4A of the tapered roller and the guide surface 8 in the collar portion of the inner ring under a high load or a high speed rotation, whereby it is hard to form a lubricating oil film. Therefore, there is pointed out a problem such as a break of the lubricating oil film, a seizure, a reduction of a seizure resistance, or the like.